Phase shifting device



Aug. 14, 1951 E. M. PURCELL 2,554,030

PHASE SHIFTING DEVICE Filed Dec. 10, 1945 FIG. 2 1

IO 2 6 18 22 Q i q INVENTOR. EDWARD M. PURCELL A T TOR/V5 Y Patented Aug. 14, 1951 PHASE SHIFTING DEVICE Edward M. Purcell, Cambridge, Mass, assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application December 10, 1945, Serial No. 634,070

3 Claims.

This application relates to electrical circuit components and more particularly to devices for producing a phase shift in signals in the ultrahigh and microwave frequency regions.

In radio and electronic devices, or systems, in which the frequency of oscillation of the energy involved is in the ultra-high or microwave regions, sections of open wire transmission line, coaxial line, or wave guide are often used as inductors, capacitors, transformers, etc. For example, short-circuited or open-circuited sections of transmission line or wave guide may be used as inductive or capacitive reactance components. To produce a desired amount of phase shift in any circuit, there may be inserted in the circuit a section of line, the electrical length of which may be varied. Such a section of line is commonly known as a line stretcher. In radio frequency systems which employ wave guides, line stretchers used heretofore have included U- shaped sections of line cumbersome in size and operation, and utilizing sliding contacts which often prove unsatisfactory.

It is an object of the present invention, therefore, to provide a unique phase shifting device or line stretcher which may be used in radio frequency systems employing Wave guides.

It is a further object of the present invention to provide a unique phase shifting device in which the phase shift introduced may be easily adjusted.

For a better understanding of the invention together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing in which:

Fig. 1 is an isometric view of the preferred embodiment of the present invention; and

Fig. 2 is a schematic illustration of the embodiment shown in Fig. 1.

Referring now to the drawing and more particularly to Fig. 1 thereof, there is shown a wave guide assembly here termed a magic-T, denoted generally by I9, comprising arms I2, I4, I5 and I8. The arms I2, I4, I6 and I8 are sections of rectangular wave guide designed to propagate the desired signal, operating preferably in the dominant mode. The arms I8 and Iii are preferably co-=linear and the arms I2 and I4 are joined to arms I6 and I3 at right angles. The longitudinal axis of the component wave guides intersect at a common point, hereinafter termed the electrical center of the magic-T. The arm I2 is joined to the arms i6 and I8 in what is commonly called a series connection. The arm IQ is joined to the arms I6 and IS in what is commonly known as a parallel connection.

The magic-T has several unique properties or characteristics which include:

(1) If arms I5 and I8 are terminated in their "haracteristic impedances and a signal is supplied to the arm I l, energy will be present in arms I6 and I8 in equal magnitude and in phase at points equidistant from the center of the magic-T. N0 energy will be present in arm I2.

(2) If energy of equal magnitude and in phase is supplied to arms I6 and I8, energy will be present in arm I4. No energy will be present in arm I2.

(3) If the arms I5 and I6 are terminated in their characteristic impedances and energy is supplied to arm I2, energy will be present in the arms I6 and I8 in equal magnitude and out of phase at points equidistant from the center of the magic-T. No energy will be present in arm I4.

(4) If energy of equal magnitude and in 180 phase opposition is supplied to arms It and I8, energy will be present in arm I2. No energy will be present in arm I4.

(5) If energy enters either arm IE or IE, energy will be present in arms I2 and I4, but no energy will be present in the remaining arm I8 or I6.

Any four-terminal network which exhibits the properties I through 5, listed above, may be termed a magic-T. For example, a section of circular wave guide designed to operate in the dominant mode, having two coupling terminals physically displaced about the periphery of the wave guide at one position along the axis thereof, and having two more coupling terminals physically displaced 90 from each other at a second position along the axis of the wave guide and respectively displaced 45 from the first two coupling terminals, possesses the unique properties listed above and is therefore another embodiment of a magic-T.

Referring again to Fig. l, mounted for axial movement within arms I6 and I8 of the magic-T It are plungers 20 and'22, respectively, which effectively provide short circuit terminations for the arms I8 and I8. Certain mechanical structure and refinements, well known in the art, may be provided in the magic-T shown in Fig. 1 to adapt it for association with a wave guide transmission line and to produce displacement of plungers 20 and 22 in a manner to be described. Fig. 1 illustrates an embodiment of the basic structure of the invention, and the manner in which it operates as a line stretcher will become apparent the description of Fig. 2- 1" ollows.

In Fig. 2 there is shown schematically the magic-T is of Fig. 1, in which the arms l6 and [8 contain the shorting plungers 2E} and 22. The plungers 2i? and 22 are at electrical distances D1 and D2, respectively, from the center of the magic-T It. The distances D1 and D2 are variable, as will appear, but at all times differ by an odd number of quarter-wavelengths. The arms !2 and [4 are to be considered connected in series with the radio frequency line in which it is desired to provide a phase shift or line stretcher. It will be assumed the magic-T is so connected into the radio frequency line that energy enters the arm It as indicated by arrow 24. This energy divides equally and in phase into the arms [6 and i8. The energy in the arms [6 and I8 undergoes complete reflection at the short circuits provided by the plungers 29 and 22. However, it will be noted that the energy travels different distances in the two arms l6 and it. In arm it, for example, if D2 is greater than D1 by an amount equal to one-quarter wavelength, the energy travels one-half wavelength farther than the energy in the shorter arm [6. The energy which reaches the electrical center of the magic-T ll] after having been returned from the short circuits 2!] and 22 will therefore arrive in 180 phase opposition. Referring to the characteristic of the magic-T set forth in (a) above, it will be see-21 that all of the energy reflected from the plungers 20 and 22 will pass into the arm 12 and none will return into the arm M.

The distances D1 and D2 may be increased. or decreased by equal amounts, to change the effective electrical lengths of the paths from a given point in the arm It to a given point in the arm 12 while maintaining a constant difference (an odd number 'of quarter-wavelengths) b tween these lengths. The phase shift in degrees thus introduced is equal to this increase or decrease, multiplied by 360, and divided by the wavelength as measured within the wave guide.

which The magic-T structure may include suitable mechanism (not shown) to move the plungers equal distances toward or away from the center of the magic-T in, this mechanism may be calibrated to indicate the length of line or amount of phase shift introduced between two given points in the arms I2 and M.

It will be realized by those skilled in the art that various refinements in the structure as illustrated in Fig. 1 of thedrawing are contemplated. Such refinements have been here omitted for the sake of simplicity. For example, no matching devices have been shown for termina ing the wave guide arms !2 and is in their characteristic impedances at their junctions with the remaining arms HS and i8. Such matching devices, and the improved performance of structures in which they are utilized, are well known in the art. Similarly, it may be desirable to utilize some form of choke joint, many of which are well known in the art, between the plungers and the walls of the wave guide, rather than simply a sliding contact as has been illustrated. Arms :2 and [4 may be associated with a wave guide transmission line in any desired manner, for example by means of wave guide elbows and choke joint coupling flanges, or by other means well known in the art.

Although the preferred embodiment of the magic-T {G is as shown in Fig. 1, there are many physical forms which a magic-T may take, and the invention is, therefore, not limited to the use of the specific type magic-T shown.

While there has been described What is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention.

What is claimed is:

1. An apparatus for providing a predetermined amount Of phase shift in a radio frequency signal passing therethrough, comp-rising a magic-T having sections of wave guide intersecting to form first, second, third and fourth arms, shortcircuit termination means within said third and fourth. arms, said termination means being so positioned along said third and fourth arms that all of the signal entering said first arm is transferred to said second arm, and means for causing equal displacement of said termination means relative to the electrical center of said magic-T, whereby an input signal applied to said first arm causes an output signal in said second arm, said output signal being shifted in phase relative to said input signal by an amount determined by said displacement.

2. An apparatus for providing a predetermined amount of phase shift in a radio frequency signal passing therethrough, comprising a magic-T having a first arm, a second arm, a third arm, a fourth arm, first and second non-dissipating impedance terminations within said third and fourth arms, said first and second terminations bein so located with respect to the electrical center of said magic-T that all of the signal entering said first arm is transferred to said second arm, and means for causing equal displacement of said terminations relative to the electrical center of said magic-T, whereby an input signal applied to said first arm causes an output signal in said second arm, said output signal being shifted in phase relative to said input signal by an amount determined by said displacement.

3. An apparatus for providing a predetermined amount of phase shift in a radio frequency signal passing therethrough comprising a magic-T having sections of wave guide intersecting to form first, second, third and fourth arms, said third and fourth arms being colinear, short circuit termination means within said third and fourth arms, the distance between the electrical center of said magic-T and said termination means in said third arm differing by an odd number of quarter-wavelengths from the distance between the electrical center of said magic-T and said terminating means in said fourth arm, whereby an input signal in said first arm causes an output signal in said second arm to be shifted in phase relative to said input signal by an amount determined by the distance of the said terminations from said electrical center.

EDWARD M. PURCELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,129,669 Bowen i Sept. 13, 1938 2,373,233 Dow Apr. 10, 1945 2,399,661 Bowie May 7, 1946 2,445,895 Tyrrell July 27, 1948 

